Flow, Turbulence and Combustion最新文献

Turbulent hydrogen flames with varying operating conditions in the swirl-axial air injection AHEAD combustor were studied computationally with a multi-regime flame closure method, combustion LES / Stochastic fields. The method was validated by comparisons with measurements in isothermal and reacting flows. The velocity fields, flames, mixing fields and thermo-chemical states were analysed in detail. Further comparisons were carried out for different operating conditions to study the effect of global equivalence ratio and axial air injection ratio. On the one hand, it introduces higher axial momentum, which restricts flashback. On the other hand, increasing the global equivalence ratio or axial air injection ratio negatively affects the spatial mixing quality where the axial momentum flux plays an important role. The results also suggest that thermo-chemical states are dominantly controlled by the global equivalence ratio rather than the inlet reactant temperature or flow conditions. The effect of differential diffusion was also studied. Differential diffusion slightly increases the possibility of the upstream occurrence of the flame inner branch, which results in the inner flame branch brush becoming broader. This was found to be related to the changes of the upstream mixing field due to differential diffusion. Nevertheless, the global system is negligibly influenced by differential diffusion due to the high Reynolds number.

Ammonia/methane ((textrm{NH}_3)/(textrm{CH}_4)) co-combustion offers a promising route toward high-efficiency, low-carbon, and low-NO_x combustion, yet experimental insights into local displacement speeds in ammonia-containing flames remain scarce. Here, 2D particle image velocimetry (2D-PIV) was applied to turbulent premixed Bunsen flames at atmospheric pressure, covering ammonia fractions up to 0.6 and equivalence ratios between 0.7 and 1.0. Streamline-based decomposition of the progress-variable transport equation showed that the measured flame propagation speed s_T is broadly consistent with the closure relation s_R + s_F at low turbulence intensities, supporting the applicability of the Bray-Moss-Libby (BML) thin-flame concept to (textrm{NH}_3)/(textrm{CH}_4) flames. The reaction and convection terms dominated the balance, while turbulent fluxes, though weaker, remained non-negligible. Closure errors increased with higher ammonia content or leaner mixtures but were greatly reduced by introducing Markstein corrections and approximating 2D flame surface density as 3D. Analysis of reaction rates further revealed that the near-axis region contributed most, while ammonia addition lowered local reactivity, increased flame height, and promoted local extinction at the root. The findings provide experimental guidance for source-term estimation and turbulence–flame interaction modeling.

Introducing a vegetation barrier in place of one building obstacle within a canonical street canyon of identical geometry yields a coupled vegetation-building (V–B) or building–vegetation (B–V) configuration, depending on whether the vegetation is located on the upwind or downwind side of the canyon, whose flow structure and turbulence characteristics remain less explored compared with the canonical building-building (B–B) case. The extent to which results in V–B and B–V canyons deviate from or converge with those of a generic B–B canyon remains unclear. Using large-eddy simulation across a range of leaf area densities (LAD), this study examines how vegetation placement and porosity modulate canyon-scale vortex geometry, turbulence structure, and air exchange. The zero-w trace delineating the primary canyon vortex, which aligns approximately with the vertical axis in a unit-aspect-ratio B–B canyon, exhibits a monotonic tilt toward the vegetation side as LAD decreases, reflecting progressive deformation and weakening of the mean circulation. Within V–B canyons, turbulence intensity and vertical momentum flux show little dependence on LAD, implying that upwind vegetation primarily acts as a momentum sink that suppresses near-wall shear. In contrast, these turbulence statistics in B–V canyons exhibit stronger sensitivity to LAD ((R^2sim 0.81) and 0.65, respectively), suggesting enhanced turbulence generation through canopy-wake interaction. Both mean and fluctuating air-exchange rates increase smoothly with LAD for vegetated canopies but decline sharply as the system approaches the impermeable B–B limit. This non-monotonic behaviour demonstrates that the (LADtoinfty) limit is dynamically distinct from an impermeable wall, as dense vegetation retains finite permeability and sustains shear-layer instabilities absent in solid obstacles.

This work assesses a POD/SVD-based sparse sensing framework for thermochemical field reconstruction in the LUCY cyclonic combustor operating under MILD methane combustion conditions. A reduced-order representation is built from CFD data and combined with QR decomposition with column pivoting (QRCP) to design sensor layouts and infer reduced coefficients from sparse measurements. Two truncation criteria are compared, the optimal hard threshold of Gavish et al. and a 99.5% cumulative variance, together with Auto-scaling preprocessing, to quantify their impact on reconstruction accuracy and out-of-sample prediction. Beyond unconstrained QRCP, practical deployment constraints are investigated through distance separation and region-limited candidate sets, and the performance of a predefined thermocouple layout is evaluated by augmenting it with additional optimally placed sensors. Results are reported for both training and testing operating conditions, complemented by an uncertainty-propagation analysis based on experimental measurement variability. The study shows that robust reconstruction is achieved with a limited number of sensors when the balance between retained rank and measurements is respected, while spatial constraints can preserve near-optimal performance and improve physical feasibility. The framework provides a computationally efficient soft-sensing strategy to support monitoring-oriented applications in confined MILD combustors.

High-speed diagnostics are essential for understanding the unsteady parameter fluctuations in Rotating Detonation Combustors (RDCs). However, the experimental data from RDCs often exhibit significant stochasticity spatially and temporally due to factors such as lap-to-lap detonation wave fluctuations, measurement uncertainties, and sensor-induced artifacts. Traditional phase-averaging techniques, like the arithmetic mean, can distort the true detonation wave structure by smoothing out key features due to temporal misalignment. This study investigates the application of a soft-Dynamic Time Warping (soft-DTW) based averaging as a unique method for processing high-speed RDC data. Compared to conventional methods, soft-DTW has shown improved resilience to local time axis distortions, which may enable better alignment and preservation of the intrinsic wave structure, particularly by capturing the sharpness of the main peak and secondary features relevant to the detonation process. The study evaluates the capability of soft-DTW to capture essential physical characteristics of rotating detonation waves using dynamic pressure and video data. Additionally, a sensitivity analysis assesses the method’s effectiveness in accurately representing secondary features, such as reflected shock waves, highlighting its potential for more representative RDC data averaging.

Direct numerical simulations of the concentric annular turbulent pipe flow are performed to investigate the skin-friction drag reduction effect by the wall oscillation control technique. The oscillation on the inner and outer walls is synchronized, and the control effect is investigated for different radius ratios. While drag reduction by wall oscillation has been extensively studied in canonical wall-bounded flows such as plane channels and circular pipes, its effects in geometrically non-uniform configurations, such as the annular pipe, remain less understood. Curvature and frictional asymmetry give rise to characteristic flow responses. The simulations demonstrate that the friction drag decreases owing to the wall oscillation. The maximum drag reduction rate is (R_D approx 0.5) at the optimal period of (T^+ approx 150). An identity equation for the skin-friction coefficient of the annular pipe flow is derived, and the contribution from turbulence is quantitatively discussed. Scaling methods for the drag reduction rate are applied: 1) increment of the mean velocity and 2) based on the Stokes problem. Both scaling methods appropriately describe the drag-reduction behavior in the drag-reduction regime at (T^+ lessapprox 150).

Hydrogen/methane (H₂/CH₄) blends serve as a transitional fuel for gas turbines, offering a pathway toward cleaner energy by reducing carbon emissions while leveraging existing natural gas infrastructure. Understanding the effects of H₂ content and pressure on the flame shape is essential for safe operation and optimising combustion performance. In this work, turbulent lean premixed Bunsen H₂/CH₄ flames are studied using large eddy simulation (LES), focusing on the flame brush length (f_l) and thickness (f_b) along the centreline. The results have been compared with available measurements for validation. The findings indicate that increasing H₂ content leads to shorter and thinner flames, while the pressure effects are minimal. At the lower H₂ content (≤30% by volume), the change in f_l is relatively small, whereas at the higher H₂ content ( > 30% by volume), f_l decreases linearly with increasing H₂ content. Similarly, the values of f_b decrease as H₂ content is increased. This phenomenon is primarily due to the increased turbulent flame speed (s_T) resulting from enhanced fuel reactivity. The limited pressure effects on f_l and f_b are attributed to the consistent s_T, maintained by the synergistic interactions between chemical reactions and turbulent characteristics. Furthermore, this study highlights the transition of the flame regime from the corrugated flamelet regime to the thin reaction zone regime along the axial direction. This transition occurs further upstream in flames with higher H₂ content, driven by the increased flame speed. Additionally, the reaction progress variable contours and profiles show that the H₂ addition also leads to a radial thinning of the flame, while pressure has little effect on the overall flame shapes.

Scramjet External Nozzle (SEN) generally has a single expansion ramp wall in one side and the other side wall called cowl is fully or partially truncated. These designs are to aim large expansion area using rear part of a vehicle airframe with reduction in system weight, friction loss, and heat load. The present study experimentally investigated the effect of cell base width on the thrust performance of the SEN and computationally expressed those effects, based on the wind tunnel test. The cell base structure divides each engine module. Three test models were employed of which shape differs from each other. One is not clustered configuration, while the others are clustered and each of them has a different cell base width. The experimental study showed that the nozzle wall pressure distribution varies corresponding to the cell base width and optimum-expansion condition is most sensibly affected by cell base width. Additionally, the input correction is proposed to reflect the clustering effect to the computation model, which unifies the input physical quantities by solving mass, streamwise momentum, and energy conservation equations. The effectiveness of such a correction was confirmed using so-called Easy-to-Handle Prediction Model (EHPM) for SEN which is based on the wave method. Comparison between calculation and validation test results demonstrated that the prediction error could be suppressed under the threshold of 10% through the proposed correction.

Direct numerical simulations with multi-step chemistry were performed for one- and two-dimensional freely propagating laminar premixed flames of methane–air and hydrogen–air mixtures with a matching density ratio to isolate the effects of hydrodynamic instability while allowing for a variable effective Lewis number, with the methane (hydrogen) flame being thermodiffusively stable (unstable). Entropy diffusion and generation mechanisms were analysed based on contributions from heat conduction, viscous dissipation, mass diffusion, and chemical reactions. Across both flames, chemical reactions were identified as the dominant source of entropy generation, with viscous dissipation contributing negligibly compared to other mechanisms. Significant differences were found in the structure of entropy generation rates across both flames, with varying degrees of correlation with curvature. Stronger correlations were found between the irreversible entropy generation rates and the heat release rate in both flames, suggesting the former as a potential marker for thermodiffusive instability. Analysis of the entropy generation profiles at representative locations across a flame front further revealed possible origins of the entropy behaviour under thermodiffusively stable and unstable conditions.